The present invention relates to semiconductor laser devices and, in particular, to a semiconductor laser device capable of realizing high power, high reliability and long life, as well as its manufacturing method, and further to an optical disc reproducing and recording apparatus.
In recent years, there have been being achieved developments in AlGaAs-based semiconductor laser devices aimed at higher power and longer life by implementing an Al-free (Al-absent) quantum well structure (well layer and barrier layer). This is because the presence of Al at an oscillator end face would cause a surface level to occur at the oscillator end face, making catastrophic optical damage (COD) to be liable to occur, which is disadvantageous for high power, long life and high reliability.
For example, Japan Journal of Applied Physics Vol. 38 (1999) pp. L387–L389 reports a semiconductor laser device having an Al-free active region. This semiconductor laser device, as shown in FIG. 11, is made up by stacking, on a GaAs substrate 301, one after another, a GaAs buffer layer 302, an Al0.63Ga0.37As lower cladding layer 303, an In0.49Ga0.51P lower guide layer 304, an In0.4Ga0.6P barrier layer 305, an In0.13Ga0.87As0.75P0.25 well layer 306, an In0.4Ga0.6P barrier layer 307, an In0.49Ga0.51P upper guide layer 308, an Al0.63Ga0.37As upper cladding layer 309 and a cap layer 310.
Generally, there is a difference in optimum growth temperature between an Al-free semiconductor layer and an AlGaAs-based layer. For example, InGaAsP or GaAsP or the like is low in growth temperature, as compared with AlGaAs-based materials. Therefore, in the case where an AlGaAs-based layer is stacked after the stacking of an Al-free semiconductor layer, it is necessary to interrupt the crystal growth after the stacking of the Al-free semiconductor layer, and then elevate the temperature before making the AlGaAs-based layer grown.
Unfortunately, since the Al-free semiconductor layer would remain exposed as a topmost surface during the interruption of crystal growth, temperature elevation would cause P to be desorbed (reevaporated), so that the Al-free semiconductor layer—AlGaAs-based layer interface would become larger in roughness.
To prevent this problem, the above-mentioned conventional semiconductor laser device is so designed that the Al-free semiconductor layer and the AlGaAs-based layer are grown continuously at a constant temperature. This provides an advantage that the crystal growth is not interrupted, thus preventing the Al-free layer from being exposed at the topmost surface during an interruption.
However, in the semiconductor laser device of the prior art as described above, the growth temperature for the continuous crystal growth of the AlGaAs-based layer is 720° C., which is a higher temperature for the Al-free semiconductor layer. It is said that the optimum growth temperature for InGaAsP is about 650° C., for example. As a result, even with the continuous growth, there has been left a problem that P is likely to be desorbed at the growth temperature of 720° C., resulting in a roughened surface. This problem has had causal connections with increased deterioration and worsened reliability of the semiconductor laser device.